This application claims priority to German application No. 198 41 559.1, filed Sep. 11, 1998, herein incorporated by reference.
1. Field of the Invention
The invention relates to (meth)acryloxy-functional polyacrylates obtained by transesterification and to their use as degassing agents for paints and coatings, especially radiation-curing coatings.
2. Description of the Related Art
The prior art discloses the use of polyacrylates of a variety of compositions as degassing agents. Homopolymers of butyl acrylate and copolymers of ethyl acrylate and ethylhexyl acrylate are used in particular for this purpose. Exemplary applications are the degassing of 2-component epoxy flooring compounds (Pitture Vernici Eur. (1997), 73, 34-38) or powder coating systems (inter alia, EP-A-0 561 543). In many cases, however, the degassing action of such agents is inadequate.
The use of relatively hydrophobic polyacrylates with higher alkyl radicals and/or additional (meth)acryloxy groups as degassing agents for paints and coatings, on the other hand, has not been described.
The invention relates to the use of such special polyacrylates for degassing coatings, especially radiation-curing (UV/EB) coatings. By adding such polyacrylates it is possible to avoid the coarsely and finely disperse air bubbles incorporated into such coatings, without adversely affecting other properties of such coatings.
The increasing demands for more ecologically acceptable, emissions-reduced coating systems which can also be processed economically brought to the fore the recent technology of UV- or EB-induced radiation curing. In this technology, systems based on free-radically curable acrylates, which are discussed in more detail below, have acquired the greatest importance. Such systems are known and are described, for example, in xe2x80x9cUV and EB Curing Formulation for Printing Inks, Coatings and Paintsxe2x80x9d (R. Holeman, P. Oldring, London 1988).
Principal binders are oligomeric acrylate compounds based on polyethers, polyesters, epoxy resins or polyurethanes. The average molar masses are customarily within the range from 200 to 4000 g/mol. The required processing viscosity is established if desired by adding low-viscosity monofunctional or multifunctional monomers, such as hexanediol diacrylate, tripropylene glycol diacrylate, trimethylolpropane triacrylate, etc., which act as reactive diluents. The curing mechanism is a radiation-induced, free-radical polymerization. In the case of UV curing, the polymerization is started by the photoreaction of an initiator. Examples of such photoinitiators are acylphosphine oxide, acetophenone and benzophenone derivatives, and thioxanthone. Amine derivatives are sometimes added as synergists for the purpose of acceleration. After coating, usually by flow, roller or spray techniques, and irradiation with UV light or electron beams, materials coated in this way can immediately be processed further or packed.
There is a causal link between the preparation of such systems, and even more so their processing properties, and the additives employed. Degassing in particular is a very critical problem, since only a few seconds elapse between application and the subsequent radiation-induced drying. Consequently, in many cases, finely (5 to 50 xcexcm) dispersed spherical air bubbles remain in the film, resulting in a distinct loss of gloss. Radiation-curing coatings of this kind possess a very low solvency, so that the addition of known degassing substances (e.g., silicone fluids or organically modified siloxanes) can very easily result in unwanted clouding, flow defects, craters, or reduction in gloss. The addition of silicone-based additives also has a strong negative impact on the overcoatability of such coatings, so preventing the construction of multicoat systems or at least making it much more difficult. The addition of silicone-based additives is particularly undesirable in the case of flow coating applications, since it is generally not possible to prevent breaks in the curtain. Additives tried and tested by those skilled in the art nowadays include the addition of small amounts of methyl ethyl ketone or butyl acetate, although this conflicts with the desire to formulate low-emission systemsxe2x80x94or, ideally, emission-free systems. These solvents must largely be removed from the film prior to irradiation.
Therefore, there is a need in the art for silicone-free additives which are easy to incorporate and which, when added at low concentrations, eliminate the microdisperse air or suppress its formation without adversely affecting other properties of the coating (gloss, overcoatability, intercoat adhesion, and resistance to solvents and water). At the same time, such additives should be largely independent of the nature and composition of the coatings to which they are added in order to improve said properties and hence should be capable of universal application.
It is the object of the invention to find compounds which meet the above requirements and are effective when added in small amounts.
This object is achieved in accordance with the invention. Accordingly, this invention provides for polyacrylates having an average molecular weight of from 1000 to 10,000 and the general formula 
where
R4 is the radical of a known chain regulator or initiator,
R1 is identical or different and is an alkyl radical,
R2 is identical or different and is a saturated or unsaturated alkyl radical of 12 to 22 carbon atoms,
R3 is a hydrocarbon radical which carries at least one (meth)acryloxy group,
a is from 10 to 50,
b is from 3 to 20,
c is from 0 to 10,
and the ratio a:b+c is from about 0.25 to about 4 and the ratio b:c is from about 1:0 to about 1:0.7. This invention also provides for the use of those polyacrylates as degassing agents for paints and coatings.
According to the prior art, compounds of this kind are obtainable by copolymerization and/or subsequent transesterification reactions. Firstly, the respective synthesis route is guided by economic considerations; secondly, however, transesterification reactions on polyacrylates having 1 to 4 carbon atoms are advantageous specifically for obtaining polymers with a narrow distribution, in the case of the desired incorporation of crosslinkable (meth)acryloxy functions, and for minimizing any residual monomer presence and associated physiological risks.
In the fairly recent past, transesterification products of this kind have been described on a number of occasions, such as, for example, in DE-A-38 42 201 or DE-A-38 42 202 and in DE-A-42 36 337. These transesterification products, for example, have significant advantages over the analogous copolymers, such as a substantially more uniform molecular weight distribution. They are largely free from monomeric fractions. Only by using the transesterification process is it possible to prepare polyacrylates whose alcoholic ester component includes unsaturated double bonds, without the formation of high-molecular-mass byproducts. For instance, it is possible without complications to transesterify polyacrylates with oleyl alcohol and at the same time with other hydroxy compounds.
Surprisingly, it has now also been found that this applies to hydroxy-functional (meth)acrylates as well. Copolymeric structures obtained in this way are novel. They are preferably obtained by polymer-analogous transesterification reactions on polyacrylates having 1 to 4 carbon atoms.
The transesterification is performed in each case on polyacrylates whose alkyl groups have preferably 1 to 4 carbon atoms. A particularly preferred alkyl group is the methyl group. The alkyl group is selected primarily in accordance with the boiling point of the corresponding alcohols.
In the case of the polyacrylates to be used in accordance with the invention, then, the transesterification component a) used comprises saturated or unsaturated alcohols of 12 to 22 carbon atoms. Particularly suitable alcohols are the saturated fatty alcohols derived from the naturally occurring fatty acids by hydrogenation, such as lauryl alcohol, myristyl alcohol, palmityl alcohol, stearyl alcohol or behenyl alcohol. Of particular interest, however, are the unsaturated alcohols, especially oleyl alcohol, which when used as transesterification component produce particularly effective degassing agents. In the case of a copolymerization, the use of oleyl acrylate as monomer is preferred.
The chain regulators and initiators provided for in the present invention are well known to those skilled in the art and such compounds may be obtained commercially or by techniques known in the art. Such compounds include, for example, alkyl mercapto compounds with n-dodecyl mercaptan being especially preferred.
The transesterification component b) can be employed additionally for the transesterification, although its use is optional and not mandatory. Its use, however, is particularly advantageous when crosslinking of the additive is desired. Suitable components b) are all hydroxy-functional (meth)acrylates, especially hydroxyethyl acrylate.
Among the formulators of radiation-curing systems, the desire for as high as possible a proportion of crosslinkable components in the formulation is becoming more and more of a priority. The use of (meth)acrylicized and thus crosslinkable polyacrylates minimizes their tendency to migrate and the proportion of substances extractable from the film. Therefore, their use is particularly preferred.
The molar ratio of the alcohols R2OH and R3OH is from about 1:0 to about 1:0.7, in particular from 1:0.1 to 1:0.5.
The transesterification is conducted with amounts of components R2OH and R3OH such that a degree of transesterification of from about 25 to about 80% is achieved. Preference is given to a degree of transesterification of from 50 to 80%.
The transesterification proceeds in the manner known per se from the abovementioned patent and application documents at temperatures from about 70 to about 140xc2x0 C. in the presence of a transesterification catalyst and in the presence or absence of a solvent.
The degassing agents to be used in accordance with the invention are added to the paints and coatings in an amount of from about 0.01 to about 5% by weight, preferably from 0.1 to 1% by weight, based on the total formulation.
The polyacrylates to be used in accordance with the invention can be employed as they are or in solution in solvents, especially reactive diluents. In accordance with the prior art, further compounding with hydrophobic inorganic or organic solids is advantageous for increasing the activity. Particular preference is given to the use of hydrophobic silica. These solids can be incorporated by dispersion in amounts of from 0 to 10% by weight into the polyacrylates to be used in accordance with the invention.
The polyacrylates of the invention are employed in particular in radiation-curing coatings. Such radiation-curing coatings, based on polyether acrylates, polyester acrylates, epoxy acrylates or polyurethane acrylates, or mixtures thereof, can comprise additives (pigments, fillers, leveling agents, etc.) as are conventional in the coatings sector. The desired application viscosity of the coatings produced with the compounds of the invention can be established by appropriately regulating the addition of reactive and/or nonreactive solvents. These coating compositions are suitable for coatings which adhere to a large number of substrates, such as wood, plastic or paper, for example. These coatings can be applied in a conventional manner, by spraying, flow coating or roller coating, for example.